Method for producing a contact layer on a silicon-containing material



Oct. 18, 1966 E. FITZER ETAL 3,279,042

METHOD FOR PRODUCING A CONTACT LAYER ON A SILICON-CONTAINING MATERIAL 2 Sheets-Sheet 1 Filed July 19. 1962 Fig- 2 Oct. 18, 1966 E. FITZER ETAL METHOD FOR PRODUCING A CONTACT LAYER ON A SILICON-CONTAINING MATERIAL 2 Sheets-Sheet 2 Filed July 19; 1962 Fig. 5

aaaaaaz-caaa'ia United States Patent 3,279,042 METHOD FOR PRODUCING A CONTACT LAYER ON A SILICON-CONTAINING MATERIAL Erich Fitzer, Karlsruhe, and Ottmar Rubisch and Friedrich Selka, Meitiugen, uber Augsburg, Germany, assignors to Siemens-Planiawerke Aktiengesellschaft fiir Kohlefabrikate, Meitingen, uher Augsburg, Germany, a corporation of Germany Filed July 19, 1962, Ser. No. 210,940 Claims priority, application Germany, July 20, 1961, S 74,922 2 Claims. (Cl. 29155.69)

Our invention relates to electrically conducting components, such as heating rods for furnaces, which consist of silicon-containing material, and are provided with an electrically conducting coating or surface layer; and our invention also concerns itself with a method of producing such surface layers on a silicon-containing material.

Silicon-containing materials are preferably employed for the production of very high temperatures. Particularly employed are electric heating elements consisting of silicon carbide or molybdenum silicide or titanium silicide or another high-melting (refractory) silicide of a transition metal from the fourth to sixth B-groups of the periodic system of elements. It is a difiicult matter to reliably provide the body of such a heating element with a contact layer as required for connecting the element to an electric cable or other electric-current supply component.

It is known to provide a heating-element body of the above-mentioned type with a contact layer of aluminum by a metal-spraying method. This, however, involves the danger that such a spray-deposited layer of aluminum, as soon as its thickness becomes greater than about 1 mm., tends to scale off or tends to exhibit in its entirety a rather poor adhesion and can be pulled off in the form of a more or less closed sleeve. A mechanical anchoring of such a metal layer by notching, ring-shaped grooves and the like in the heating-element body can mechanically prevent such pulling of the contact sleeve from the element body. This, however, does not suflice to secure a fast broad-area adherence as required for a good transfer of electric current. It has also been proposed to subject aluminum layers, sprayed or extruded onto the silicon-containing body, to mechanical deformation for the purpose of improving the area adherence.

Detrimental to all mechanical expedients, however, is another phenomenon. That is, molybdenum silicide as well as other silicides tend to form relatively large quantities of siO -cont aining oxidation products at temperatures considerably lower than the incandescent temperature to which the heating elements must often be subjected in normal operation. These oxidizing temperatures in many cases are only a few hundred degrees centigrade. In the case of MoSi for example, the oxidation products contain silicic acid as well as molybdic acid. The latter acid is known to have a very intensive tendency for growth. When the aluminum contact layer is not firmly area-joined with the silicidecontaining heater-element body, the formation of the oxidation products results in a gradually increasing separation and loosening of the aluminum layer from the silicide-containing body and ultimately in an interruption of the electric contact.

It is an object of our invention to eliminate these difi'iculties and to provide a method for obtaining a reliable area contact between the silicon-containing material and the metallic contact layer While affording a Patented Oct. 18, 1966 ice greatly increased life of useful operation of the resulting products.

To this end, the method according to the invention is partly similar to the above-mentioned known methods in that it serves for producing a contact layer of aluminum or other metallic material having a similar melting point, on a silicon-containing material consisting particularly of silicide carbide or a high-melting (refractory) silicide, preferably of molybdenum silicide or titanium silicide. However, the above-mentioned shortcomings and difficulties are eliminated according to the invention by applying the contact layer in two steps or strata. First, the contact location in cold condition is metallized, thus being coated with a thin layer whose thickness is about 0.1 to 0.5 mm. preferably. Thereafter, a second layer of greater thickness is deposited upon the first metal layer and clansists of a contact material having the same or a higher melting point than the contact material used in the underlying metallizing coating, the second layer being deposited in molten condition.

According to another, preferred, feature of our invention, the first coating is deposited by metal spraying, for example with the aid of a spray nozzle or extrusion nozzle, and then depositing the enveloping cover layer by casting it in molten form about the first deposited coating.

Another way of proceeding according to the invention is to deposit the first metallization of the contact area by applying the contact material electrolytically, by vapor deposition, thermal dissociation, or by cathode sputtering. Common to all these methods of deposition is the fact that the contacting area remains in cold condition, so that the formation of an oxide layer on the surface of the silicon-containing material is avoided. This is of utmost importance for thev desired result of the method according to the invention.

The novel method according to the invention, despite its particular simplicity, reliably eliminates the above-mentioned difficulties heretofore encountered. If the underlying metal coating on the contact area of the silicon-containing body is deposited by the metal-spraying method, this method is preferably performed by subjecting the contact area of the silicon-containing body to grinding so that it assumes a smooth surface condition. Then, with the body in cold condition, a thin aluminum layer is sprayed onto the contact area. During spraying, the metal being sprayed solidifies even while traveling through the air, and its kinetic energy causes it to become plastically deformed when impinging upon the body, thus securing a firm adhering pressure of the aluminum particles against the body of the heater element. As long as the spray-deposited coating remains thin, it secures an excellent electrical contact. However, while such a thin coating suffices for establishing a good electric contact, it is in itself insufficient for reliable attachment of a terminal or other current conductor. This possibility, however, is afforded by the second, thicker layer which according to the invention is deposited in molten and hence liquid condition of the contact material. As mentioned, it is preferable to cast the cover layer about the spray-deposited thin layer. If for this purpose the contact location of the silicide-containing body is heated, an oxide coating will be formed, but now this oxide coating does not occur on the silicide-containing surface of the heater-element body proper but rather on the surface of the spray-deposited aluminum layer. The oxide layer forming at this location is not detrimental. When it is contacted by the molten aluminum forming the second layer, the underlying spray-deposited aluminum layer becomes dissolved and liquefied and thus becomes bonded with the melt of aluminum being cast about the body under conditions which cause removal of the oxide skin previously formed on the underlying aluminum layer. Any oxidic residue may then appear as a scum on the outer surface of the outer layer.

The same result can be obtained by substituting the casting of aluminum by immersion of the initially coated body into a mass of molten aluminum, or by embedding the initially aluminum-coated body in comminuted aluminum such as shavings, granules, or grains which thereafter are heated to the melting point of this embedding mass of aluminum.

During subsequent cooling, the metal envelope produced in any of the ways according to the invention described above contracts to a greater extent than the siliconcontaining material of the heater element. Consequently, an elastic deformation of the metal envelope now takes place. As a result, an electric contact with the heater body is secured of such a good mechanical quality that it remains preserved even after repeated, relatively slight heating of the contacted envelope during operation of the heater element.

The invention will be further explained with reference to the embodiments of heater elements according to the invention illustrated by way of example on the accompanying drawings in which:

FIG. 1 is a plan view of a complete heater element.

FIGS. 2, 3 and 4 show, partly in section, the ends of respectively different heater-element bodies; and

FIG. is a lateral view of a contacted end of a heater element body with an electric cable attached thereto.

The main portion of the heater element shown in FIG. 1 is constituted by the heater element proper and consists of the silicide-containing material, preferably of molybdenum silicide or titanium silicide. The main portion has a hairpin-shaped major part 1 of smaller cross section than the respective ends 2 and serves to assume glowing temperature during operation. The parts 2 consist of the same material and are integral with the part 1 but, due to their larger cross section, assume lower temperatures during operation. The two end parts 2 are followed by the contact portions 3 which are continuations of the low-temperature parts 2 and consist of the same silicide-containing material as the parts 1 and 2. However, the parts 3 are enveloped in a contact layer and their respective ultimate ends 4 each form a terminal for connection to a cable or other conductor 5 consisting, for example, of aluminum wire strands. A connector, such as a terminal screw or bolt, joins the cable 5 with the end portion 3. The outer end of each cable 5 is joined with a cable shoe 6 for accommodating a pressure screw or the like attaching means for connection of the heating element to current-supply busses.

The operating temperature of the glowing part 1 of smaller cross section is at about 1700 C. The low-temperature part 2 is then at temperatures between 300 C. and 700 C., naturally with a gradual transition zone between the above-mentioned high temperature to the lower temperature. When the element is in operation, the contact part 3 protrudes out of the furnace in which a heater element of the illustrated type is being used. Consequently, the part 3 has a still lower operating temperature than the low-temperature part 2 of the element.

FIGS. 2 and 3 show on larger scale only one end of the heater-element body in order to illustrate in which particular manner the contact layer is joined with the body of the element.

According to FIG. 2, the end 10 of the heater element has a symmetrical shape of circular cross section. When producing the element, a thin aluminum layer of about 0.1 to 0.5 mm. thickness is spray-deposited at 11. Thereafter the second layer 12 is deposited. For lucidity of illustration, the two layers 11 and the thicker layer 12, the latter being about 2 mm. thick, are shown as separate strata. In reality, however, the original spray-deposited coating 11, as explained above, is melted when the outer layer 12 is cast about the end of the heater element so that the two layers 11 and 12 become more or less coalesced and form a single unitary layer. A decisive advantage is the fact that after completion of this contact layer, no oxide layer is interposed between the contact layer and the end 10 of the heater element, because no oxide skin is formed when the metal layer 11 is being sprayed onto the cold heater element; whereas any oxide layer occurring during heating of the element body, already provided with the layer 11, occurs on this layer 11 and is dissolved and floated away when the layer 12 is cast about the body. For that reason, the method according to the invention reliably produces a sufliciently thick and strong contact layer of excellent adherence to the silicon-containing material of the heater element 10 proper.

In the embodiment according to FIG. 3, the end of the silicide-containing body of the heater element to be provided with a contact layer is also of circular cross section but is given a shape which, seen from the glowing part of the element, first tapers toward the axis of the body and thereafter again widens. Placed upon this part 20 of the element body is a thin aluminum layer 21 of essentially uniform thickness. Thereafter the portion 20 and the spray-deposited layer 21 of the heater element are surrounded by casting with an envelope 22 of aluminum whose outer surface is cylindrical. Relative to the coalescence of the layers 21 and 22, the foregoing explanation of layers 11 and 12in FIG. 2 is also applicable to the embodiment of FIG. 3. The drawing here again shows the two layers are separate strata only for the purpose of lucid illustration. With respect to the electric current transfer, the embodiment of FIG. 3 also corresponds to that of FIG. 2. In addition, however, the embodiment of FIG. 3 affords the advantage of better resisting any mechanical pull in the direction of the longitudinal axis of part 20 and layers 21, 22. Consequently, the envelope 22 cannot be stripped off for mechanical reasons of structure as well as of metallic bonding.

The embodiment of FIG. 4 generally corresponds to that of FIG. 2. The free end of the heater-element body. is denoted by 30. The spray-deposited thin layer of metal is designated as 31, and the thicker contact layer, cast about the layer 31, is denoted by 32. The part 30 is provided with regularly or irregularly shaped or distributed recesses 30a preferably of semispherical configuration. The thin metal layer 31 is spray-deposited upon the surface of the part 30 with substantially uniform thickness throughout. The contact layer 32 is cast about the body and is given an external cylindrical surface. In such a design, the recesses in the body of the heater element take. care that the contact sleeve 32 cannot be stripped off even if a strong pulling force is applied.

As mentioned, aluminum or an aluminum alloy is par ticularly suitable for use in the spray-deposited layer as well as in the surrounding cast layer. However, instead of aluminum, other contact materials can also be used which have a melting :point not too far remote from that of aluminum or aluminum alloys, in comparison with the.

melting or operating temperature of the silicon-containing material. The first layer and the second layer, that is, in the preferred embodiments the spray-deposited inner layer and the outer or cast layer, can also be made of re-.

spectively different contact materials so chosen that, the first-deposited layer serving for metallizing the contact end of the heater-element body, may melt at least superficially when the second, thicker layer is being cast about the inner layer, so that any oxide coatings which may have been formed when the contact end was heated during the casting operation, are floated away.

The silicon-containing body of the heater element is very brittle. If a reliable electric contact engagement is to be obtained, it is not only necessary to secure a reliable electric contact between the end of the element body and a metal layer as described above, but it is also desirable to take care of a good connection of the cable, terminal or other circuit component that must be connected with the element. It has been found that if such a current-conducting component is directly clamped to the contact end of the element, the brittle body of the element may easily be cracked or broken. Furthermore, such heater elements may have low-ohmic resistance and operate with relatively high current intensities. Under such conditions, there is the danger that the current supplying components may be subjected to considerable heating, which also requires taking care of providinga permanently reliable contact connection.

FIG. 5 illustrates an embodiment which satisfies the just-mentioned requirements and desiderata by forming a suitable supplementation of a contact surface on a heating element produced and designed in accordance with the invention. According to this embodiment, the contact end of the element body is flattened as indicated by the separation line 40 in FIG. 5. These angularly related lines 40 represent respective planes that are both perpendicular to the plane of illustration. The contact end of the silicon-containing body, here denoted by 41, is flattened at the location area in accordance with these two boundary planes 40, the flattening being done by machining, for example.

Deposited upon the contact end 41 of the element body, thus flattened at 40, are the two above-described metal layers 42 and 43. A cable 44 of aluminum strand is equipped with a cable shoe 45 preferbaly consisting also of aluminum. The cable shoe 45 is in good electrical contact with the bare ends of the wire strands of the cable 44. The shoe 45 has the nesting shape apparent dirom FIG. 5 and contacts the flattened area. Consequently, when mounted together with the heater element, the shoe 45 supplements the flattened end of the element so as to form a full cylinder together therewith. Thus joined 1 together with the heater element, the cable shoe 45 is fastened together with the contact end of the element body 41 by an aluminum rivet 46. Another way of joining the cable shoe 45 with the element body 43 is by cold welding or by upsetting under pressure. All of these joining methods are known and available as permanent connections as contrasted to removable screw or clamp connections.

It will be obvious to those skilled in the art, upon a study of this disclosure, that this invention permits of various modifications and alterations with respect to the individual components and arrangements disclosed, and hence can be embodied in equipment other than as particularly illustrated and described herein, without departing from the essential features of the invention and within the spirit and scope of the claims annexed hereto.

We claim:

1. The method of producing an electric resistance heater element which comprises forming indentations in the ends of a silicide heating rod, flame spraying a first coating of aluminum to a thickness of about 0.1 to about 0.5 mm. thickness on said ends and then casting a thicker second coating of aluminum on said ends while removing the 0X- ide layer that had formed on the initial coating and solidifying said composite coating.

2. The method of producing an electric resistance heater element which comprises forming indentations in the ends of a silicide heating rod, flame spraying a first coating of aluminum to a thickness of about 0.1 to about 0.5 mm. thickness on said ends and then casting a thicker second coating of aluminum on said ends while removing the oxide layer that had formed on the initial coating and solidifying said composite coating whereby the composite casting forms a current-supply terminal means of the heater element.

References Cited by the Examiner UNITED STATES PATENTS 1,787,749 1/ 1931 Heyroth 29-15571 1,906,963 5/1933 Heyroth 29-155.71 X 2,475,379 7/1949 Strong 338-309 2,629,166 2/1953 Marsten et al. 29-155.7 2,820,534 1/1958 Hume 29-4731 X 2,863,034 12/1958 Tassara 338-309 3,013,328 12/ 1961 Beggs 29-155.7 3,100,338 8/1963 Henry 29-4731 FOREIGN PATENTS 541,816 12/ 1 Great Britain.

JOHN F. CAMPBELL, Primary Examiner.

RICHARD M. WOOD, WHITMORE A. WILTZ,

Examiners.

V. Y. MAYEWSKY, P. M. COHEN, Assistant Examiners. 

1. THE METHOD OF PRODUCING AN ELECTRIC RESISTANCE HEATER ELEMENT WHICH COMPRISES FORMING INDENTATIONS IN THE ENDS OF A SILICIDE HEATING ROD, FLAME SPRAYING A FIRST COATING OF ALUMINUM TO A THICKNESS OF ABOUT 0.1 TO ABOUT 0.5 MM. THICKNESS ON SAID ENDS AND THEN CASTING A THICKER SECOND COATING OF ALUMINUM ON SAID ENDS WHILE REMOVING THE OXIDE LAYER THAT HAD FORMED ON THE INITIAL COATING AND SOLIDIFYING SAID COMPOSITE COATING.
 2. THE METHOD OF PRODUCING AN ELECTRIC RESITANCE HEATER ELEMENT WHICH COMPRISES FORMING INDENTATIONS IN THE ENDS OF A SILICATE HEATING ROD, FLAME SPRAYING A FIRST COATING OF ALUMINUM TO A THICKNESS OF ABOUT 0.1 TO ABOUT 0.5 MM. THICKNESS ON SAID ENDS AND THEN CASTING A THICKER SECOND COATING OF ALUMINUM ON SAID ENDS WHILE REMOVING THE OXIDE LAYER THAT HAD FORMED ON THE INITIAL COATING AND SOLIDIFYING SAID COMPOSITE COATING WHEREBY THE COMPOSITE CASTING FORMS A CURRENT-SUPPLY TERMINAL MEANS OF THE HEATER ELEMENT. 